Field of the Invention
The present invention relates to a speech coding apparatus for digitalized speech signals, more particularly, relates to a speech coding apparatus which operates under the vector quantization control method.
In the usual digital speech transmission system, analog speech signals are sampled at a sampling frequency of 8 kHz, converted to digitalized input speech signals of 8 bits every one sampling, and transmitted at a speed of 64 kbps. Various types of compression means have been proposed for reducing this transmission rate (64 kbps). For example, there is known the gain (magnitude) shape (phase) vector quantization method which can reduce the rate to 8 kbps or 4 kbps or so.
The above-mentioned gain shape vector quantization method is extremely effective as a high quality speech coding method and may be expected to be utilized for the following systems:
1) Intracompany digital communication systems, PA1 2) Digital mobile radio communication systems (car telephones), PA1 3) Speech information storing and answering systems.
In systems such as the above using the high quality speech coding method, it is necessary that the hardware be made as small in size as possible and that the quality of the speech reproduced on the receiver side not be allowed to deteriorate.